In a known brake booster of the kind specified (DE-A 42 11 849 A1) the stopper element is secured to the piston and extends outwardly through radial recesses of the valve barrel and valve body in alignment with each other where it coacts with a stopper formed on the booster housing. The recess of the valve barrel is dimensioned so that its resting position is dictated by the valve barrel being in direct contact with the stopper element. The seal interposed between valve barrel and piston is a lip-type seal held in an annular groove of the piston and, irrespective of the position of the valve barrel with respect to the piston, is in sealing contact with a cylindrical inner wall of the valve barrel. This results in friction with the drawback that solenoid actuation of the valve barrel as well as its subsequent return to its rest position are obstructed. Reliable return of the valve barrel to its rest position following exclusively solenoid actuation, i.e. with the piston halted in its rest position necessitates powerful dimensioning of the return spring which, in turn, together with the aforementioned friction forces makes solenoid actuation of the valve barrel difficult. This is why the solenoid also needs to be dimensioned powerfully which adds to its space requirement--already substantial in the cramped space availability within the booster --still further, and also results in heat dissipation problems due to the added current consumption of the solenoid.
In another known brake booster of the kind specified (EP 0 478 396 A1) the piston is configured with a coupling element which in the rest position of the piston is axially spaced away from an abutment assigned to it and is formed on the valve barrel. In mechanical actuation of the brake booster the piston must thus be shifted forwards by a certain distance before its coupling element comes up against the abutment of the valve barrel and reliably coupling it into the movement. It may, however, also happen that the valve barrel is coupled into the movement right from the start by friction transmitted from the piston via the seal to the valve barrel. Although this has no damaging effect, here too, the friction has the drawback already described.
The invention is thus based on the object of maintaining the solenoid forces required in a pneumatic booster with solenoid auxiliary control as low as possible.
This object is achieved according to the invention by the features of claim 1.